3.1 Cardiology and Resuscitation Overview

ECG Interpretation: A Systematic Approach

Cardiology and Resuscitation is a high-yield clinical domain on the NREMT Paramedic cognitive exam (roughly 10-14% of items, of which 30% are graphical ECG strips) and rewards candidates who interpret rhythms in a fixed, repeatable order rather than pattern-matching. Read every strip the same way: rate, rhythm/regularity, P waves, PR interval, QRS width, ST-segment/T-wave changes.

Rate. On standard paper (25 mm/s) each large box is 0.20 s. Use the 300-150-100-75-60-50 sequence (count large boxes between two R waves) for regular rhythms, or the 6-second method (count QRS complexes in 6 s, multiply by 10) for irregular rhythms. Normal sinus rate is 60-100; under 60 is bradycardia, over 100 is tachycardia.

Regularity. March out the R-R intervals. Regularly irregular suggests patterned conduction (e.g., Wenckebach); irregularly irregular strongly suggests atrial fibrillation.

P waves and PR interval. One upright P before every QRS with a constant PR of 0.12-0.20 s defines sinus rhythm. A prolonged PR (over 0.20 s / one large box) is first-degree AV block.

QRS width. Normal QRS is under 0.12 s (three small boxes). A wide QRS (0.12 s or more) means the impulse originated below the AV node or is conducted aberrantly.

Axis in one line

For field purposes, glance at leads I and aVF: both QRS complexes upright = normal axis; I up / aVF down = possible left axis deviation; I down / aVF up = right axis deviation. Axis is rarely the answer by itself but supports rhythm calls (e.g., extreme axis with a wide-complex tachycardia favors VT).

The Four Arrest Rhythms

Memorize which rhythms are shockable. Only ventricular fibrillation (VF) and pulseless ventricular tachycardia (pVT) respond to defibrillation; asystole and pulseless electrical activity (PEA) never get shocked and instead drive a search for reversible causes (the H's and T's).

Confirm asystole in two leads and verify lead/gain before calling it, because fine VF can masquerade as a flat line.

Common Dysrhythmias

• Atrial fibrillation - irregularly irregular, no discrete P waves, often rapid ventricular response; chronic AFib raises stroke risk.
• Atrial flutter - sawtooth flutter waves, frequently a 2:1, 3:1, or 4:1 conduction ratio (atrial rate near 300).
• SVT (AVNRT) - narrow, regular, very fast (150-250), P waves usually buried.
• First-degree AV block - PR over 0.20 s, every P conducts.
• Second-degree Mobitz I (Wenckebach) - progressively lengthening PR until a QRS drops; usually benign.
• Second-degree Mobitz II - constant PR with intermittently dropped QRS; high risk of progressing to complete block.
• Third-degree (complete) block - P waves and QRS march independently (AV dissociation); the escape rhythm sets the ventricular rate.
• Monomorphic VT - wide, regular, fast; treat as VT until proven otherwise.

Exam trap

A wide-complex regular tachycardia in an older patient with cardiac history is ventricular tachycardia until proven otherwise. Do not assume SVT with aberrancy and reach for a calcium-channel blocker; that error can be fatal. When in doubt and the patient is unstable, the answer is electricity (synchronized cardioversion), not a diagnostic drug.

Connecting Rhythm to Action

The exam rarely asks you to name a rhythm in isolation; it asks what you do because of the rhythm. Build the reflex: every interpretation ends in a decision. Sinus tachycardia at 130 in a febrile patient means treat the cause (fluids, cooling), not slow the heart. Atrial fibrillation with a rapid ventricular response that is causing hypotension means the patient is unstable and needs synchronized cardioversion, whereas the same rhythm in a stable patient means rate control. A new third-degree block in an inferior MI means prepare to pace.

When you see a P wave for every QRS and a QRS for every P, the AV node is conducting one-to-one; problems live in the rate or the underlying cause. When P waves and QRS complexes lose their relationship, you are dealing with a block, and the grade of the block determines urgency. A wide QRS always raises two questions: is this a ventricular rhythm, or is it a supraventricular rhythm conducted with a bundle-branch block or aberrancy? In the unstable patient you do not need to resolve that debate before acting.

Reading artifact versus rhythm

Before treating a lethal-looking strip, rule out artifact. A patient who is awake, talking, and has a strong pulse is not in VF no matter how chaotic the tracing - check leads, motion, and the patient first. Likewise, a flat line may be a disconnected lead, asystole, or fine VF; confirm asystole in a second lead and turn up the gain. Treating the monitor instead of the patient is one of the most punished errors in this domain, on the exam and on the street alike.